1. Field of the Invention
This invention relates to impact printing and in particular to impact printers in which dots are recorded on a print medium to form images, lines, symbols or the like.
2. Discussion of the Prior Art
In a dot matrix printer, which in some cases may also be referred to as an all-points addressable dot printer, individual dots are recorded selectively at all addressable point positions in a continuous line of dots extending across a record medium. In order to produce recorded images of good print quality, the recorded dots must be precisely located and uniformly spaced at all addressable points of the line and it is desirable to be able to record successions of spaced dots as closely together as possible.
In the multiple blade and helix printers of the type for printing characters, a separation or gap exists between the print elements or type-carrying elements to permit interference-free individual operation. To enhance interference-free individual operation, the prior art shows the use of an over-under hammer and print element structure. This type of structure generally involves the use of carrier elements having engraved characters on the front side and projections alternatively arranged on two levels on the backside, with respect to the print medium. The carrier elements are mounted for movement along a print line. Hammers for striking the projections are arranged in superimposed fashion in two rows, one row being on an upper level and the other row being on a lower level.
U.S. Pat. No. 3,698,529 describes a back-printer using a stationary interposer with over-under projections that are acted on by hammers which move on a carriage and uses a wheel containing engraved characters which moves along with the hammers to accomplish serial printing. This arrangement would be unsatisfactory for use as a dot matrix line front printer which must print using a plurality of hammers simultaneously. Also, there is difficulty in moving the relatively massive hammers.
U.S. Pat. No. 3,719,139 describes a high-speed printer wherein the type-carrying member is provided with staggered rows of projections (over-under) on the reverse side of each type character and wherein a plurality of hammers operating at a print position selectively cooperate with the projections of a respective row. Both the type carrying members and the hammers are moved and the printing elements are presented in sequence to carry out serial printing. This arrangement would not be adaptable for use as a dot matrix printer wherein many positions are printed simultaneously.
U.S. Pat. No. 3,773,161 describes a high speed on-the-fly serial printer wherein character printing is accomplished one character at a time along a succession of printing positions. A print carriage is movable from one printing position to the next. A pair of printing hammers are mounted on the printing carriage and they are arranged on two levels so that the two printing heads are partially overlapping. This arrangement would also be unsatisfactory for use as a dot matrix line front printer which must print many positions simultaneously. Also, it is desirable to have hammers which are stationery.
The over-under hammer and print element structures in the above prior art are satisfactory to permit interference-free individual operation. The blade separation presents little problem for character printing since such printing naturally requires some separation between characters for legibility. However, in the all-points addressable printing of dots, the present invention uses a plurality of fixed cantilevered hammer elements which co-act with a plurality of cantilevered print elements which are mounted on a reciprocating shuttle. With the use of a reciprocating shuttle, it became apparent that an over-under arrangement for the hammer elements and print elements was required which would prevent interaction between adjacent print elements and which also would prevent the crashing between impactor bars and impact receiving bars when the shuttle reached the end of its movement in one direction and then reversed to move in the opposite direction.